定义

表示委托，委托是一种数据结构，它引用静态方法或引用类实例及该类的实例方法。 Delegate 类是委托类型的基类。然而，只有系统和编译器可以显式地从 Delegate 类或 MulticastDelegate 类派生。此外，还不允许从委托类型派生新类型。Delegate 类不是委托类型，该类用于派生委托类型。

问题

该类型不支持 直接使用 deleagte 关键字赋值实例

如下

this.Invoke(delegate
                {
                    this.textBox1.Text = num.ToString();
                });

上述代码 为winform中窗体类的成员方法， 其不能通过编译 ， 无法将 匿名方法 转换为类型“System.Delegate”，因为它不是委托类型

Google结果

The problem the user is seeing is that the Thread ctor accepts a specific delegate – the ThreadStart delegate. The C# compiler will check and make sure your anonymous method matches the signature of the ThreadStart delegate and, if so, produces the proper code under-the-covers to create the ThreadStart delegate for you.

But Control.Invoke is typed as accepting a “Delegate”. This means it can accept any delegate-derived type. The example above shows an anonymous method that has a void return type and takes no parameters. It’s possible to have a number of delegate-derived types that match that signature (such as MethodInvoker and ThreadStart – just as an example). Which specific delegate should the C# compiler use? There’s no way for it to infer the exact delegate type so the compiler complains with an error.

对于Thread.ctor()来说，由于接受的是一个ThreadStart委托，编译器便可以将匿名函数与ThreadStart委托类型匹配，最后能够正确编译。
而对于Control.Invoke()来说，任何的代理类型都是可接受的，也就是说ThreadStart和MethodInvoker都是可以接受的类型。这样编译器反而不知道应该用哪个代理去匹配匿名函数了，导致了编译错误的发生。

引用自http://www.cnblogs.com/yelaiju/archive/2010/09/16/1827691.html

解决方案

Invoke(new MethodInvoker(delegate { this.textBox1.Text = num.ToString();}));
Invoke(new MethodInvoker(() => { this.textBox1.Text = num.ToString();});

this.Invoke((ThreadStart)delegate
                {
                     this.textBox1.Text = num.ToString();
                });